Question

${{10}^{-4}}$mg of gelatin is required to be added to $100\text{c}{{\text{m}}^{3}}$ of a standard gold solution to just prevent its precipitation by the addition of 1 \[\text{mL}\] of $10%$ of $\text{NaCl}$ solution is added to it. Hence the gold number of gelatin is?
A.$10$
B.\[1.0\]
C.$0.1$
D.$0.01$
E.$0.001$

Answer 1

Hint: The knowledge of colloids is important to solve this question. Gold number is defined as the minimum mass of the colloid in milligram that is added to that is added to 10 mL of red gold sol to protect it from coagulation when 1 mL of $10%$ of $\text{NaCl}$ solution is added to it.

Complete step by step answer:
Here the colloid is “Gelatin” whose gold number is to be determined. According to the question,${{10}^{-4}}$ of gelatin is required to be added to $100\text{c}{{\text{m}}^{3}}$of a standard gold solution.
As 1 \[\text{mL}\]=1 $\text{c}{{\text{m}}^{3}}$ so, $100\text{c}{{\text{m}}^{3}}$=100 \[\text{mL}\]
As per the definition, the gold number is a measure of the mass of colloid in 10 \[\text{mL}\]of red gold sol so for 100 \[\text{mL}\] solution, the gold number should be,
$ \Rightarrow \dfrac{{{{10}^{ - 4}} \times 10}}{{100}}$
$ \Rightarrow {10^{ - 5}}{\text{ g}}$ = ${10^{ - 2}}{\text{ mg}}$ = $0.01{\text{ mg of gelatin}}$

Hence option D is correct.

Note:
-A colloid is formed when positively and negatively charged particles form a bigger molecule just like how blood coagulates.
-Lyophilic colloids are those that have more affinity towards the dispersion phase and are more stable in water.
-The protective power of a colloid is determined by the reciprocal of the gold number and according to Zsigmondy the smaller is the value of the gold number more will be its protective power to prevent the coagulation.
-Gelatin is considered to be the best protective colloid because the value of the gold number for it is the smallest, i.e.,$0.01$.